1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a laser beam printer and the like, and more particularly relates to an image forming apparatus which is capable of conveying a recording medium to an image transfer unit with favorable accuracy.
2. Related Background Art
Conventionally, an electrophotographic type image forming apparatus such as a copying machine and a laser beam printer and the like forms a toner image on a photosensitive member drum and then allows a transfer unit to transfer the formed toner image onto a recording medium which is being conveyed to the transfer unit.
Such a conventional image forming apparatus has a guide member extending up to the vicinity of the photosensitive member drum, for reliably guiding the recording medium to a transfer portion at which the toner image is transferred, as shown in Japanese Patent Application Laid-Open (JP-A) No. 2002-255401.
FIGS. 6 and 7 are diagrams explaining a construction of a guide according to JP-A No. 2002-255401. As shown in FIGS. 6 and 7, a latent image is formed on a photosensitive member drum 501, and then the formed latent image is subjected to toner development by a development unit 502. Then, the toner image is transferred onto a recording medium which is being conveyed to a transfer unit 504, which constitutes a nip portion (transfer nip portion) in association with a transfer roller 503, to thereby record the image by applying a transfer bias to the transfer roller 503.
Moreover, between a conveyance roller pair 505 for conveying the recording medium, and the transfer unit 504, an upper transfer guide 506 and a lower transfer guide 507 are disposed to reliably convey the recording medium to be conveyed, to the transfer unit 504.
The transfer unit 504, the upper transfer guide 506, and the lower transfer guide 507 are mounted to a conveyance frame 508. This conveyance frame 508 is pivotably supported on a front side plate and a rear side plate (both not shown) with a supporting point as a center, and then the development unit 502 is slidable in the directions of an arrow m (left and right directions) in FIG. 6.
FIG. 6 shows a state in which the transfer roller 503 abuts on the photosensitive member drum 501, and FIG. 7 shows a state in which the conveyance frame 508 is pivoted downward to retract the transfer roller 503 from the photosensitive member drum 501, and the development unit 502 is slid to a retracted position.
The upper transfer guide 506 and the lower transfer guide 507 are arranged at locations such that their tip portions stand very close to the photosensitive member drum 501 with a gap of about 1 to 3 mm in order to reliably guide the recording medium to the transfer nip portion. Particularly, the upper transfer guide 506 is disposed at a location which is closest to the photosensitive member drum 501, which enables a positional relationship between the photosensitive member drum 501 and the upper transfer guide 506 to be ensured with a high degree of accuracy.
[Patent Document] Japanese patent publication No. 2002-255401A
According to the prior art, the development unit 502 is slidable in the left and right directions in FIG. 6, as described above, and the upper transfer guide 506 which is disposed in the vicinity of the development sleeve 502a in the development unit 502 is fixed to the conveyance unit 508; therefore, many components should be interposed between the development sleeve 502a and the upper transfer guide 506, which requires the upper transfer guide 506 to be arranged away from the development sleeve 502a by a distance which is set by taking the variation of the component dimensional accuracy into consideration.
However, making a gap between the upper transfer guide 506 and the photosensitive member drum 501 wider disables the recording medium to be smoothly guided to the transfer nip portion, thereby enabling the recording medium to be conveyed to the photosensitive member drum 501 with a touch of abutment, which causes the shock resulting from the recording medium abutting on the photosensitive member drum 501 to influence the image. Furthermore, making the gap excessively wider may result in defective conveyance.
On the contrary, when misalignment between the upper transfer guide 506 and the photosensitive member drum 501 causes a tip portion of the upper transfer guide 506 to get too close to the photosensitive member drum 501, the toner adhered to a surface of the photosensitive member drum 501 immediately before the transfer is readily adhered to the tip portion of the upper transfer guide 506, thereby causing the adhered toner to be adhered to the recording medium, which may deteriorate the quality of image.
Therefore, it has not been easy to mount the upper transfer guide 506 on to the photosensitive member drum 501 or the development sleeve 502a with favorable accuracy, taking the variation of the component dimensional accuracy into consideration.